Generally, data transmission systems convey information in the form of packets from an originator to a specified addressee. Typical systems, such as token ring and polling systems, query the specified addressee to determine if the addressee has information to be transmitted. These types of systems, however, are not without their limitations. The token ring data transmission system, for example, passes a token or message to all users around a ring to determine if the users have data to be transmitted. Each user on the ring receives the message, decodes it, and transmits data if necessary. Polling systems are similar in that a periodic poll is sent to all users to determine if data transmission is required. The users again decode the poll and transmit data if necessary. Both of these data transmission systems limit system throughput by introducing unnecessary data on the bus or lines carrying the data. The systems also tie up the users when transmission may not be necessary.
Data can also be transferred in time-division multiplexed (TDM) systems, such as satellite communication systems. Satellite systems generally use a satellite acting as a relay, receiving and transmitting requests from land-based ground stations. When the ground stations require data transmission, they request access to a TDM time-slot. Depending on the prioritizing mechanism, the requests are processed and the TDM time-slot issued. The system, however, is limited in the fact that the satellite system acts on a distributed queue management basis, assigning all necessary time-slots to the requesting ground stations at the start of a TDM frame. This requires that all ground stations receive the request information and monitor when other ground stations transmit, and when their particular transmit time arrives, access the system. If one of the requesting ground stations does not receive any of the requests sent by other ground stations, it has the possibility of becoming out of synchronization with the rest of the system. If this happens, the ground station must first re-synchronize to the system and repeat the requesting process. The process of re-synchronization and repeating the requesting process can be very time consuming and result in perceptible user delay.
The systems mentioned may also perform scheduling of channels, but not on a frame-by-frame basis. The satellite system, for example, is required to keep track of all the ground stations that have requested access and can thus schedule the ground station access in time. If, as mentioned above, a ground station becomes out of synchronization with the system, the scheduled timeslots in the corresponding frames go un-used since the ground station is not able to gain access. This method of scheduling yields an inefficient use of the TDM frame timeslots when ground stations lose synchronization with the system.
Thus, a need exists for a data transmission system which can schedule timeslots in a TDM system on a frame-by-frame basis and also look ahead in time and pre-schedule data channels to reduce processing time and aid in transmission efficiency.